1. Field
The present disclosure is generally related to security marks on documents.
2. Description of Related Art
Security marks, barcodes, and glyphs that are considered invisible to the eye are well known, and are used for many applications, such as tracking workflow and job security, high-security tickets, high-value documents and anti-counterfeiting applications. Some of these security marks are made with materials (e.g., ink or toner) which have almost no absorption in the visible spectrum of light, but have significant absorption in non-visible parts of the electromagnetic spectrum (i.e., to the human eye), such as ultraviolet (UV) or infrared (IR) radiation. Other marks rely on fluorescence of the material under suitable light, such as UV radiation.
Some of these applications use approximately “colorless” rather than truly “invisible” inks or toners. Such colorless materials are commonly “clear” inks/toners, i.e., materials similar to other inks/toners used in the system, except that they are substantially free of any colorants such as pigments or dyes, which give the other inks/toners their characteristic color properties in the visible range. These clear inks/toners typically have refractive indices and surface characteristics that are different from the substrate. In many marking technologies, such as xerography, lithography, flexography, and UV inkjet and solid inkjet technologies, the inks/toners exist in one or more layers on top of the paper. Consequently, the clear inks/toners can be detected by human observers, even without any special equipment, due to the gloss difference between the applied ink/toner and the bare substrate. For example, in some cases, when one alters the viewing angle of a page, the gloss of the invisible mark can be seen. For some applications this may not matter, but where truly invisible security marks are required, these materials are inadequate.
This gloss differential is a problem for such materials. In some cases, it has been possible to match closely the gloss of an ink or toner with a gloss of a particular substrate (e.g., uncoated paper) that the ink/toner is applied on. However, even though it may be possible to match the gloss of a particular ink to one particular substrate, developing and using a different ink/toner for each substrate is inconvenient and not necessarily feasible. For example, the ink/toner selected most likely will not match the gloss on other papers or substrates (e.g., gloss coated paper). Generally, the ink or toner can have a gloss appearance very different from that of the default paper or other papers that may be used in the printer.
Additionally, the above matching approach would require different inks or toners to be made (and then installed in the printer) for each substrate. This is also not a practical and/or feasible solution.
Thus, although attempts have been made to match the gloss of inks and toners to that of the substrate, such an approach has limited success.